This invention relates generally to gas turbine engines, and more specifically to gas turbine exhaust gas ducts or diffusers.
Gas turbine engines and the like typically include an exhaust duct or diffuser downstream of the last stage of the turbine section. Generally described, the diffuser converts the kinetic energy of the hot flow gases exiting the last turbine stage into potential energy in the form of increased static pressure. The diffuser directs the hot flow gases through a casing of increasing area in the direction of flow. The diffuser generally includes a number of struts mounted onto a hub and extending radially to an outer casing, but other configurations are also known. Oftentimes, the struts will have an airfoil configuration.
During partial load operations, a last stage bucket exit tangential flow angle (swirl) may increase and may lead to flow separation on the struts and the hub of the diffuser. Flow separation and an increase in swirl, in turn, may reduce the diffuser static pressure recovery. More specifically, flow separation at part-load occurs almost uniformly from the center hub to the tips of the airfoil-shaped struts on their respective suction sides. Such a reduction in static pressure recovery may have an impact on overall gas turbine engine performance and efficiency.
It is also known to provide a number of smaller struts or vanes extending from the hub, such that the smaller struts alternate circumferentially with the main struts, such as shown in U.S. Published Patent Application 2013/0170969.
Traditionally, the smaller struts, which may also be airfoil-shaped, have been positioned at an angle to the main struts, but this configuration does not provide an optimum part-load operating condition. Thus, even with the utilization of the smaller struts/airfoils, an undesirable wake and separation pattern remains.
It would therefore be desirable to provide an exhaust gas diffuser that provides the desired part-load performance without otherwise incurring a diffuser and/or turbine performance penalties under other operating conditions.